A frequency duplexed system provides a common form of bi-directional communication. In a frequency duplexed system, separate frequency bands are used for the transmission of information flowing in opposite directions of a bi-directional link. FIG. 1 shows how a network of such links is typically employed to provide radio frequency (RF) distribution. A bi-directional central station receives all signals from the outside world to be distributed over the network. The network might, for example, provide radio coverage within various rooms of a building structure. Likewise all signals that are initiated at the network's bi-directional links (called remote stations), are fed through the network to be re-broadcasted to the outside world through the central station. Further, intermediate stations provide bi-directional branching points within the network. Such a system is highly advantageous for providing wireless two-way communication service especially within structures or around other obstacles, man-made or natural, which otherwise tend to block or disrupt radio waves.
It is evident that such networks can get relatively large with extensive branching and having many remote stations. This can make it difficult to locate sources of faults or problems when the network is not working properly. For example, when a component within an intermediate station or a remote station malfunctions it can cause problems which can propagate throughout large portions of the network. Ideally, one would like to have an efficient fault detection scheme that would detect faults and indicate which station has the faulty component. Once isolated, that faulty station can be replaced (more often than not, it is most cost effective to replace a broken station rather than spend time and money to find and fix a component within a circuit board of a station). The fault detection system should be easy to implement and independent of the specific realization of the communication network.
Such an error detection scheme should be inexpensive to incorporate and require minimal or no additional cabling. For example, suppose a bi-directional communication system were implemented over a 10 base T cable. The cable is inexpensive, is common and pre-existing to much building infrastructure and contains four twisted pair cables, two of which are used for bi-directional communication. For this system, an error detection scheme which could be implemented over one of the remaining twisted pair cables would be highly desirable as no new cabling would need to be installed. For this to be done with one cable it is desirable, if not necessary, that each station report its status (operability) over one wire. Or better yet a fault detection method should not even need this extra wire, but make use of the up- and down- link wires to simultaneously flag errors.
Although the prior art has addressed fault detection for distributed communication systems, the solutions tend to be complicated and, hence, expensive and in some respects incomplete. For example, in U.S. Pat. No. 4,733,223, Gilbert describes a method for monitoring the operational status of CATV components remotely located along a cable system. Each remote component is given a fixed address code. Using this address, a head-end system individually polls the remote components to check their status. When polled, the remote component reports its status using a low-frequency (LF) modulated signal superimposed on the power line. This LF signal is received and re-transmitted by upstream components located between the polled component and the head-end system. However, in this system the cable signals travel only downstream, not bi-directionally, and the status signals are specially coded and travel upstream. Furthermore, each component is responsible for determining its own fault status, yet Gilbert does not teach how this is done.
In U.S. Pat. No. 3,733,430, Thompson and Schoenbeck teach a method for remotely interrogating and monitoring the operation of television sets in a cable TV system for the purpose of identifying the on/off time of the television set and the channel to which the set is tuned. Furthermore, the invention aims to provide such interrogation and monitoring in a relatively short period of time. The operation of this invention uses FM modulated signals to transmit digital information. The digital information includes an address for the purpose of selecting individual remote sites. Each remote site must store and, when requested, transmit any required data. The signals transmitted to and received from the remote sites are separated in frequency and only one address may be interrogated at a time in this architecture. Again such a system is complicated and expensive, and not directed towards bi-directional, frequency duplexed communication system. In addition, this patent does not teach how a malfunctioning TV set or remote station appropriately discovers its faulty condition. Ideally, a malfunctioning station should automatically report its status without having to be polled.
U.S. Pat. No. 3,586,968 provides a means for determining the physical location of faults along a transmission line with multiple repeaters placed along the line. The invention is specifically intended for use in a cable PCM (Pulse Code Modulated) communication system. However, the existence of a fault must be know a priori, e.g., the fault could be made known by a customer complaint. The invention works by having a pulse generator at each possible fault point (each repeater) send back pulses to the original transmission source. The transmission source then detects the amplitude and time delay of the pulse in order to determine the fault location.